Needless to say, C is powerful. But, if you use Java or Python, you will reconize that C lakes a bunch of Libraries(APIs) or a framework for C programmer to do job quicker. Sometimes you have to look for some C libraries to meet your requirement, and then you can avoid from carving the same wheels again and again. This document will record the useful libraries for C language and I will continue to add the new one on it. For those who are a great C programmer, if you know a good library for C, please also let me know that. Thanks in advance.
OGDF - Open Graph Drawing Framework
http://www.ogdf.net/ogdf.php
Curl Lib
the multiprotocol file transfer library
http://curl.haxx.se/libcurl/
mongoose
The lightweight web server in C
http://code.google.com/p/mongoose/
SimCList – A C library for Lists
http://mij.oltrelinux.com/devel/simclist/
JSON Library
http://www.digip.org/jansson/
The libevent API provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout has been reached. Furthermore, libevent also support callbacks due to signals or regular timeouts
http://libevent.org/
libev - a high performance full-featured event loop written in C
It is similar with libevent, but is more efficiently
http://doc.dvgu.ru/devel/ev.html
The Better String Library
http://bstring.sourceforge.net/
Unit Test Frameworks
https://github.com/imb/fctx
Exception Handling for C
http://code.google.com/p/exceptions4c/
SSL Library
https://polarssl.org/ssl-library
libssh2 is a client-side C library implementing the SSH2 protocol
http://www.libssh2.org/
MD5
http://256.com/sources/md5/
CIDR Library ( Need to verify )
http://www.over-yonder.net/~fullermd/projects/libcidr
NETCONF library in C
https://code.google.com/p/libnetconf/
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine.
http://www.sqlite.org/